The present invention is directed to a process for forming thioglycolic acid in high yields by reacting mixtures of monochloroacetic acid and dichloroacetic acid with an alkali metal hydrosulfide under conditions described herein below.
Thioglycolic acid is a highly desired product having various commercial applications. It forms the active ingredient of most hair waving preparations. It is also used extensively in cosmetic depilatories and in commercial agents used to remove hair from hides and the like. Various other commercial uses are made of thioglycolic acid in addition to those described above. The large and varied demands for thioglycolic acid provide a great need for forming this material in the most effective and economical method.
Thioglycolic acid is presently prepared commercially by reacting pure monochloroacetic acid with sodium hydrosulfide in an aqueous medium. After acidification, thiogylcolic acid is extracted from the reaction mixture with some suitable organic solvent and purified by distillation. This process was first described by Carius in Ann. 124, 43 (1862) and later improved by Klason and Carlson in Ber. 39, 732-738 (1906). The later authors taught that the process is best carried out using a sodium hydrosulfide liquor of not more than 15 percent by weight and a weak monochloroacetic acid solution containing the acid in not more than 20 percent by weight.
Alternate methods have been suggested. These include a two step synthesis of initially reacting monochloroacetic acid with sodium thiosulfate followed by hydrolysis of the resulting Bunte salt (See U.S. Pat. Nos. 2,594,030; 2,413,361 and British Patent No. 624,568). Another proposed synthesis (German Patent No. 180,875) involves the reaction of monochloroacetic acid with an alkali polysulfide and then reducing the formed dithiodiglycolic acid by means of zinc and mineral acid. U.S. Pat. No. 3,860,641 described a process of forming thioglycolic acid by heating an alkali hydrosulfide with a xanthogenic acid.
Recent patents directed to modifications of the Klason and Carlson process for forming thioglycolic acid include U.S. Pat. Nos. 3,927,085 and 4,082,790. U.S. Pat. No. 3,927,085 describes a modification which requires the presence of aqueous sodium or ammonium hydroxide, a partial pressure of hydrogen sulfide and pure monochloroacetic or monochloropropionic acid in 4-12 mol. percent.
U.S. Pat. No. 4,082,790 teaches that mono, di or tri mercaptans of the formula R(SH).sub.a can be formed by reacting the corresponding mono, di or tri chloride or bromide, R(X).sub.a with H.sub.2 S and ammonia or an amine in aqueous systems to give the corresponding mercaptan R(SH).sub.a. R can represent an aliphatic carboxylic acid and a represents an integer of 1 to 3.
The process most commonly used is that described by Klason and Carlson. The starting carboxylic acid is pure monochloroacetic acid either as the free acid or as its alkali metal salt. The acid feed is required to be free of polyhalo acids to avoid the formation of the corresponding polythiols as suggested in U.S. Pat. No. 4,082,790. In view of the fact that the chloroacetic acid is normally formed as a mixture of mono and dihalo acids, the presently known art requires separation and purification of the monochloroacetic acid prior to its use in order to achieve high yields of the desired product.
It would be highly desired to have a process of forming the desired thioglycolic acid product which utilizes a combination of mono and polychloroacetic acids as feed reactant. Such a process would provide a more effective and economical mode of producing the desired product.